Continuous method for reusing coating powder waste and coating powders thus obtained

ABSTRACT

Process for working up powder coating waste to yield reusable powder coatings by compacting the powder coating waste without complete melting to yield a sintered product, in which process powder coating residues arising during powder coating production are continuously separated, continuously compacted and the compacted product is continuously ground together with fresh material to be ground of the same batch of powder coating without extrusion to yield a powder coating.

This invention relates to a process for the continuous working up andreuse of ground powder coating waste, which substantially comprisesby-products from powder coating production, for the production of powdercoatings without loss of quality.

Since no solvent emissions occur on application, crosslinkable powdercoatings are an environmentally friendly alternative to liquid lacquersystems containing solvent. As a result, the use of powder coatings hasrisen sharply in recent years.

Crosslinkable powder coatings conventionally consist of one or morepolymeric binders, curing agents, pigments and extenders together withadditives. The powder coating production process may be divided into thefollowing steps:

1. the constituents of the powder coating are vigorously premixed in therequired quantity ratios as dry solids.

2. the mixture is melted in an extruder at the lowest possibletemperature in order to avoid premature crosslinking and vigorouslymixed. This mixing plasticises the binder and curing agent and wets thepigments and extenders.

3. the resultant, optionally coloured extrudate is rolled out into athin layer, cooled and broken into coarse pellets.

4. the pellets are ground in a mill to form the finished powder coating.

In general, the fine fraction of a particle size of <10 μm, which areunavoidable in the grinding process, are removed in a subsequentscreening process. The resultant powder typically has an averageparticle size of 40 to 70 μm.

Powder coatings are generally produced in a batch process. Conventionalcompositions and the classic production process for powder coatings aredescribed, for example, in the monograph The Science of Powder Coatingsvol. 1 and 2 (ed. D. A. Bate, London 1990).

The aim of the grinding process is to produce a powder having a narrowparticle size distribution and the smallest possible content of finefraction, as a large content of fine fraction has a negative impact onthe processability of the powder coating by electrostatic spraying. Ingeneral, the fine fraction of a particle size of below 10 μm areseparated by suitable processes, for example using a cyclone. These finefractions conventionally amount to approx. 2 to 5 wt.%. If the averageparticle size of the powder is reduced, which is frequently preferredfrom a technical standpoint, there is a sharp increase in the content offine fraction with a particle size of <10 μm, as grinding always givesrise to a relatively wide particle size distribution. Powder coatingshaving a low average particle size and a narrow distribution of thegrain size range are desirable, for example, for producing thin lacquerlayers (B. Fawer, Powder Coating, October 1996, p. 56).

The fine fraction from the grinding process is accordingly a by-productin the production of powder coatings and in many cases must be disposedof as industrial waste.

DE 4028567 A1 describes the recirculation of the overspray arisingduring use of powder coatings and the mixing thereof with the startingmaterials for a new batch of powder coating. This mixture is thenfurther processed using conventional extruders. This processing involvesfusing the constituents, in particular also the overspray, and exposingthem to elevated temperatures. This process has the disadvantage thathomogeneous mixing is difficult due to the small particle size of theoverspray, problems occur with feeding the mixtures into the extruderand some fractions of the resultant powder coating are repeatedlyexposed to elevated temperatures.

WO 96/15891 describes a process which avoids the problem of metering andfeeding fines by compacting the fine powder in a tabletting press. Theresultant tablets are then mixed with the remaining starting materialsfor a new batch of powder coating and introduced into the extruder. Oneessential condition which is stated for compacting is that the originalparticles must be “substantially still discernible” in the tablets.

EP 0 683 199 A2 describes a process for the recirculation of finefraction into the starting mixture in powder coating production beforethe extrusion step by thermal sintering of the powder by distributingthe fines on a surface, agglomerating them thereon, whereupon they maybe apportioned to the starting mixture in a suitable quantity asrelatively large particles.

A feature common to all these processes is that the fine fraction orpowder residues are recirculated to the first stage of powder coatingproduction. The material has to pass again through the entire powdercoating production process. This approach has the disadvantage inprinciple that the material is again melted and exposed to elevatedtemperatures in the extruder. Since, in many cases, powder coatings arethermally crosslinking systems, repeated extrusion degrades the qualityof the powder coating, especially if a large proportion of fines isused.

DE-C 19703376 describes a process for reusing finely divided powdercoating residues, in which the powder coating residues are agglomeratedand returned to the powder coating production process. The extrusionstep may optionally be omitted in this process and grinding may beperformed directly to yield reusable powder coatings.

The batchwise working up method described in DE-C 19703376 may result inknown logistical problems relating to the necessary storage andtransport conditions. The agglomerated powder coating, for exampleproduced from overspray residues, is generally supplied to the powdercoating manufacturer by the powder coating user in order to be reused.

The object of the present invention is accordingly to provide asimplified process for working up ground powder coating waste whichfacilitates the reutilisation thereof and avoids the stated logisticalproblems. The process is intended to give rise to a homogeneous powdercoating of undiminished quality.

It has been found that this object may be achieved by the processprovided by the present invention for working up powder coating waste bycompacting the powder coating waste without complete melting to yield asintered product, wherein the powder coating waste is continuouslyseparated in the powder production process after the grinding step,continuously compacted, optionally broken up and continuously groundtogether with fresh material to be ground of the same batch of powdercoating without extrusion to yield a powder coating.

The present invention also provides the powder coatings obtainable fromthe process according to the invention.

In the process according to the invention, powder coating waste, inparticular the fine fraction arising in the grinding process, arecontinuously separated, for example using a cyclone or filter, directlycompacted and the compacted product reintroduced into the mill,preferably together with the original coarse pellets from the samebatch.

This method has the advantage that the fine fraction is directlyreincorporated into the same batch. There are consequently no problemswith colour deviations and the known logistical problems of adiscontinuous method are avoided.

The continuous mode of operation moreover makes it possible to keep thepowder coating waste in a sealed system. Especially when using the finefraction, this is advantageous for reasons of occupational hygiene andquality assurance. The problems described in DE-C 19703376 relating toconveying the fine fraction, for example in pipework, are also avoided.

The process according to the invention is thus particularly advantageousfor the continuous production of powder coatings having a small averageparticle size and narrow distribution, as large quantities of finefraction are produced in such processes which may be directly reused.

For example, it is possible to produce a powder having an averageparticle size (d50) of 30 μm, which after the classifying operation hasa content of fine fraction of <10 μm of less than 15%, in particular ofless than 10%, without significant quantities (<2%) of fine powder beingproduced.

If such a particle size distribution is to be achieved without using theprocess according to the invention, it is necessary to separate approx.12% of fine powder of <10 μm and dispose of it or reincorporate it in acomplex manner in the extruder for the production of a new batch of thesame product. Continuous compaction of the fine fraction thus permitsconsiderable savings of raw materials, as in this example, for a givenquantity of powder coating, approx. 10% less material to be ground needbe produced by extrusion. It is also possible to produce powders havingan average particle size of <30 μm, for example of 20 to 25 μm. In thiscase, the quantity of fine fraction to be compacted increases sharply,which results in a higher throughput and energy consumption of thecompacting apparatus, but has no negative impact on the quality of thepowder coating.

The process according to the invention may be used for any desiredpowder coatings, for example for clear powder coat or for colouredpowder coat, for example based on epoxy, polyester, polyurethane oracrylate resins.

Compacting conditions may be selected as a function of the type ofpowder coating used. If compaction is weak, as is for example intendedin the process for the production of tablets analogously to WO 96/15891,the compacted material breaks back down into the initial fine powder inthe mill. If the material is too strongly compacted, frictional heatingbrings about an excessive rise in temperature, which results in thepowder largely being melted. The melted powder sticks firmly to thecompacting apparatus and may only be removed again with considerableeffort. Moreover, the powder is exposed to very high temperatures, suchthat the quality of the resultant powder coatings may be impaired. In.the process according to the invention, compaction is intended toproceed in such a manner that the fine fraction is sintered togetherwithout the powder grains being completely melted and that the sinteredparts are broken up into new grain structures on grinding.

Optimum compacting conditions are dependent both upon the powder coatingused and upon the compacting apparatus. During compaction, an elevatedtemperature arises on the surface of the roll presses, which causes thepowder grains to sinter or agglomerate. In the case of powder coatingswhich crosslink at low temperature, the processing force may, forexample, be adjusted such that the individual powder grains sintertogether effectively but do not melt. In the case of powder coatingswhich crosslink at higher temperatures, a higher force and thus alsoconsequent temperature may, for example, be set. It is optionallypossible to provide the compacting apparatuses with external heating orcooling, for example the rolls may be cooled with water.

Any known compacting apparatuses are, in principle, suitable for theprocess according to the invention, such as ram presses, ram extruders,in particular two-roll presses or ring-roll presses. In this case, thecompression force in the roll presses is determined by the gap, therotational speed and quantity of feed material. The parameters mayreadily be adjusted relative to each other. The compression force isstated as a specific compression force in N per cm of roll width. Ingeneral, the specific compression force should be greater than 4 kN/cm,with a value of above 6 kN/cm in particular being favourable. Specificcompression forces are, for example, in the range from 10 to above 30kN/cm and up to 100 kN/cm. Excessive compression force may be recognisedfrom the powder largely becoming melteded, i.e. the composition stringsand sticks to the rolls. Excessively low compression forces give rise togreatly increased fines content in the grinding process. Two-rollpresses having textured surfaces have proved particularly suitable,those having tablet-shaped indentations being less suitable than thosehaving a corrugated surface.

In the event that compaction yields product which, by virtue of itsdimensions, is unsuitable for a grinding operation, the product may bebroken up, for example to form chips or pellets. Breaking may beperformed with conventional apparatus usable for powder coatingproduction, for example using so-called pin crushers.

The compacted product is continuously returned together with freshmaterial to be ground into comminution apparatuses conventional forpowder coatings, such as for example impact mills or classifier mills,which permit the production of the powdered coatings under gentleconditions. This gives rise to the further advantage of the processaccording to the invention which is that working up proceeds within theconventional powder coating production process and requires noadditional grinding plant. Before classification, the powder coatingsobtained from the compacted pellets have a fine fraction contentcomparable to that of conventional powder coatings, the fine fractioncontent for example usually being below 25%, optionally even below 15%.

The mode of operation according to the invention provides a processwhich avoids the disadvantages. of the known prior art in thereprocessing of fine powders and other powder coating waste, inparticular additional heating or melting steps. Moreover, the storageand conveying of fine fraction from powder coating production areavoided. The fine fraction arising in particular in the grinding processduring powder production may be worked up using the method according tothe invention by compaction and grinding to yield a high grade powder,without an extrusion step being required. Neither the grain size rangeof the resultant powder nor the quality of the coatings produced fromthe powder differs from conventional powder coatings.

It is even possible to produce powder coatings having a narrower grainsize range and a smaller average particle size, without there being anyneed to dispose of large quantities of by-product or to perform costlyre-extrusion.

FIG. 1 shows an example of the process according to the invention usinga block diagram. The powder coating pellets originating from the powderproduction process pass from a storage tank (1) into a mill (2) and areground therein to form the finished powder coating. The powder coatingis then passed into a cyclone (3) to separate unwanted grain sizes, forexample the fine fraction. The finished powder coating is packaged (4),while the fine fraction pass via another cyclone or a filter (5) toseparate the exhaust air stream into the compactor (6). After thecompaction process, the compacted material is broken up into a coarsematerial to be ground (7) and this material is returned, for example bymeans of pneumatic conveying (8) into the storage tank (1), where it ismixed with fresh powder coating pellets and reground to yield powdercoating.

The following Examples illustrate the invention. The compression forceof the rolls is stated as a specific compression force per cm of rollwidth (kN/cm).

EXAMPLE 1

18 kg of fine fraction of <10 μm (average grain size approx. 4 μm) fromthe grinding of a conventional thermally crosslinkable white powdercoating based on an epoxy/polyester hybrid (49% polyester, 21% epoxy,29% titanium dioxide and 1% additives) are compacted at a compressionforce of 8 kN/cm using a Bepex two-roll compactor with a roll having atextured surface at a throughput of approx. 25 kg/h. The resultantcompacted homogeneous strand, which is mechanically stable, wascomminuted in a crusher into chips of approx. 2 cm in diameter. Thesechips are ground in a mill (model ACM 2, manufacturer Hosokawa) usingthe conditions conventional for powder coatings. A powder is obtainedhaving an average particle size of 45 μm and a fraction of <10 μm of<15%.

The powder was electrostatically sprayed using a corona gun ontoaluminium sheet and stoved for 20 minutes at 180° C. A defect-freecoating is obtained having properties which do not differ from those ofa coating which is obtained directly with the above-mentioneduncompacted powder coating.

EXAMPLE 2

16 kg of fine fraction of <10 μm (average grain size approx. 4 μm) fromthe grinding of a black pigmented thermally crosslinkable epoxy-basedpowder coating with a dicyanogen crosslinking agent are compacted at acompression force of 14 kN/cm using a Bepex two-roll compactor with aroll having a textured surface at a throughput of approx. 22 kg/h. Theresultant compacted homogeneous strand, which is mechanically stable,was comminuted in a crusher into chips of approx. 2 cm in diameter.These chips are ground in a mill (model ACM 2, manufacturer Hosokawa)using the conditions conventional for powder coatings. A powder isobtained having an average particle size of 37 μm and a fraction of <10μm of <14%.

The powder was electrostatically sprayed using a corona gun ontoaluminium sheet and stoved for 20 minutes at 180° C. A defect-freecoating is obtained having properties which do not differ from those ofa coating with a powder which has not been worked up.

EXAMPLE 3

20 kg of fine fraction of <10 μm (average grain size approx. 4 μm) fromthe grinding of a conventional thermally crosslinkable polyester-basedpowder coating with triglycidyl isocyanurate (TGIC) as crosslinkingagent (55.8% polyester, 4.2% TGIC, 29% titanium dioxide and 1%additives) are compacted at a compression force of 20 kN/cm using aBepex two-roll compactor with a roll having a textured surface at athroughput of approx. 20 kg/h. The resultant compacted homogeneousstrand, which is mechanically stable, was comminuted in a crusher intochips of approx. 2 cm in diameter. These chips are ground in a mill(model ACM 2, manufacturer Hosokawa) using the conditions conventionalfor powder coatings. A powder is obtained having an average particlesize of 27 μm and a fraction of <10 μm of <25%.

The powder was electrostatically sprayed using a corona gun ontoaluminium sheet and stoved for 20 minutes at 180° C. A defect-freecoating is obtained having properties which do not differ from those ofa coating with the uncompacted powder coating.

EXAMPLE 4

16 kg of fine fraction of <10 μm (average grain size approx. 4 μm) fromthe grinding of a grey pigmented, wax-modified, thermally crosslinkableepoxy-based powder coating with dicyanogen as curing component arecompacted at a compression force of 16 kN/cm using a Bepex two-rollcompactor with a roll having a textured surface at a throughput ofapprox. 27 kg/h. The resultant compacted homogeneous strand, which ismechanically stable, was comminuted in a crusher into chips of approx. 2cm in diameter. These chips are ground in a mill (model ACM 2,manufacturer Hosokawa) using the conditions conventional for powdercoatings. A powder is obtained. having an average particle size of 36 μmand a fraction of <10 μm of <20%.

The powder was electrostatically sprayed using a corona gun ontoaluminium sheet and stoved for 20 minutes at 180° C. A defect-freecoating is obtained having properties which do not differ from those ofa coating which is obtained directly with the powder coating.

COMPARATIVE EXAMPLE 1

10 kg of fine fraction similar to Example 1 are compacted in a Bepexroll compactor with tabletting rolls to form tablets 10 mm in diameterand 3 mm in thickness, the force applied with the rollers being 4 kN/cm.Subsequent grinding of the tablets in a mill (model ACM 2, manufacturerHosokawa) yielded a powder with an average particle size of <10 μm. Thispowder behaves like the introduced fine fraction and cannot be processedin conventional powder coating processing plant.

EXAMPLE 5

8 kg of fine fraction from Example 3 are compressed with anAlexanderwerk two-roll compactor (model WP 50N 75) with a compressionforce of 9 kN/cm of roll width and a throughput of approx. 80 kg/h toyield a mechanically stable, homogeneous strand, which is coarselycomminuted mechanically and then ground in a mill (model ACM2,manufacturer Hosokawa) under the conditions conventional for powdercoatings. A powder is obtained having an average particle size of 35 μmand a fraction of <10 μm of 19%.

EXAMPLE 6

9 kg of fine fraction from Example 3 are compressed with anAlexanderwerk two-roll compactor (model WP 50N 75) with a compressionforce of 12 kN/cm of roll width and a throughput of approx. 50 kg/h toyield a mechanically stable, homogeneous strand, which is coarselycomminuted mechanically and then ground in a mill (model ACM2,manufacturer Hosokawa) under the conditions conventional for powdercoatings. A powder is obtained having an average particle size of 32 μmand a fraction of <10 μm of 15%.

COMPARATIVE EXAMPLE 2

5 kg of fine fraction from Example 3 are compressed with anAlexanderwerk two-roll compactor (model WP 50N 75) with a compressionforce of 4 kN/cm of roll width and a throughput of approx. 100 kg/h toyield a mechanically stable, homogeneous strand, which is coarselycomminuted mechanically and then ground in a mill (model ACM2,manufacturer Hosokawa) under the conditions conventional for powdercoatings. A powder is obtained, more than 40% of which consists ofparticles of <10 μm and which is unsuitable for use as a powder coating.

EXAMPLE 7

In a plant according to FIG. 1, 581 kg of fresh material to be ground ofa white pigmented powder coating based on 50:50 epoxy/polyester areground in a Hosokawa ACM 20 classifier mill and passed into a cycloneseparator to separate the fine fraction. The fine fraction passes intoanother cyclone separator and thence into an Alexanderwerke WPN 50N75roll compactor. Once the compacted material has been broken up with apin crusher, the resultant product is returned to the mill holding tankwith powder coating from the same batch. The operating conditions of themill and the two cyclones are selected such that a powder coating havingan average grain size of 28 μm and a fine fraction content of <10 μm of<15% is obtained.

The compactor was operated at a specific compression force of 21.3kN/cm. 555 kg of powder complying with the specification are obtained(96% yield).

COMPARATIVE EXAMPLE 3

In a similar manner to Example 7, the same material to be ground isground under identical conditions, but without compaction andrecirculation of the compacted material. When 594 kg of material to beground are introduced, 520 kg of powder complying with the specificationare obtained (88% yield).

What is claimed is:
 1. A process for reprocessing powder wastes from apowder coating production to provide a re-usable powder which comprisesa) continuously removing powder coating wastes arising during powdercoating production, b) continuously compacting the powder wastes using apress selected from the group consisting of a two roll press and a ringroll press, without completely melting the powder, to provided asintered product, and c) continuously grinding the compacted sinteredproduct with fresh powder coating to yield a powder composition withoutpassing said powder coating composition through an extrusion process;wherein the powder coating composition has a particle size distributionin which at least 50 percent by volume of the powder particles have aparticle size of 20 to 45 μm and in which a proportion that has aparticle size of less than 10 μm is equal to or less than 15 percent byvolume.
 2. A process according to claim 1 wherein the powder wastecomprises excessively finely divided fractions of powder obtained in theproduction of powder coating.
 3. A process according to claim 1 whereinthe compacting is performed by a two-roll press.
 4. A process accordingto claim 3 in which the two-roll press uses a specific compression forcebetween 4 kN/cm and 100 kN/cm.
 5. A process according to claim 3 inwhich the two-roll press uses a compression force between 5 kN/cm and 50kN/cm.
 6. A process according to claim 1 wherein the particle size ofthe powder waste used has an average diameter of <10 μm.
 7. A powdercomposition obtained by the process of claim
 1. 8. A process accordingto claim 1 wherein the compacting is performed by a ring-roll press. 9.A process according to claim 8 in which the ring-roll press uses aspecific compression force between 4 kN/cm and 100 kN/cm.
 10. A processaccording to claim 8 in which the ring-roll press uses a compressionforce between 5 kN/cm and 50 kN/cm.